Prior to this commit, errors unhandled by custom `WebExceptionHandler`
and resulting in an HTTP 500 status would not be logged at ERROR level,
giving no information to developers about the actual exception.

This commit ensures that such exceptions are logged at the ERROR level
with their exception. By the time the exception hits the
`DefaultErrorWebExceptionHandler`, if the response is already committed
or if the exception is due to a client disconnecting, the error is
delegated to Framework support as Spring Boot won't be able to render an
error page as expected.

Fixes gh-15769

See gh-15873

* pr/15873:
  Polish

Closes gh-15873

* pr/15871:
  Fix assertion

Closes gh-15871

Fixes gh-15790

See gh-15869

See gh-15868

See gh-15867

See gh-15866

See gh-15865

See gh-15864

See gh-15863

See gh-15862

* pr/15853:
  Polish contribution
  Polish

Closes gh-15853

See gh-15853

Closes gh-15847

Closes gh-15815

Closes gh-15843

Closes gh-15844

Closes gh-15761

Closes gh-15810

* pr/15754:
  Polish contribution
  Polish SessionConfigurationImportSelector.selectImports()

Closes gh-15754

See gh-15754